1. Field of the Invention
The present invention relates to a phosphor mixture used for an illumination device such as a light emitting diode (LED), a fluorescent display tube, and a fluorescent lamp, and a light emitting device using the phosphor mixture.
2. Description of the Related Art
As a light emitting device conventionally used as an illumination device, a discharge-type fluorescent lamp and an incandescent lamp and so forth are given as examples. However, in recent years, a white LED lighting unit using a light emitting diode (referred to as LED in some cases hereafter) that emits white light has been developed. Compared with the conventional light source for illumination, the white LED lighting unit serves as an ideal illumination device having advantages that the heat generation is reduced, an emission efficiency is excellent, power consumption is reduced, composed of only LED element or LED element and a phosphor, having long service life without blowing filament as is seen in the conventional incandescent light bulb, and capable of eliminating a harmful material such as mercury having an influence on the environment, which is indispensable to the conventional fluorescent lamp.
As required elements for a light source for a general lighting unit such as the aforementioned white LED lighting unit, the elements of luminance and color rendering properties are given as examples. The color rendering properties are values expressing a reproducibility of color by the light source, and generally, JIS Z 8726(1990) is defined as an evaluation method of the color rendering properties. Therefore, hereafter, the color rendering properties will be explained by using the evaluation method of the JIS Z 8726.
According to the JIS Z 8726, the color rendering properties of the light source is expressed in a numeric value by a general color rendering index (Ra). This is the value by which the difference between the color of a color rendering reference sample when illuminated by a sample light source, and the color of the reference sample when irradiated with a reference light analogous to a natural light is evaluated. When there is no difference between above two colors, showing absolutely the same color, the color rendering index is expressed by 100. Even if having the same correlated color temperature of the light source, the way of looks of color due is varied according to the color rendering index, and if the color rendering index is low, the color due looks darker and dull. Therefore, the light source having an uniform emission intensity over the whole visible light region can emit light close to a reference light and can be the light source with excellent color rendering properties.
At present, one of the examples of the white LED lighting unit developed as a general lighting unit functions to obtain an emission that looks white for human eyes, by combining the LED emitting blue color and the phosphor emitting yellow color under an excitation of the blue emission, and synthesizing the blue emission and the yellow emission. However, in this system, since the emission that looks white is formed of the lights of blue color and yellow color, a problem involved therein is that the lights of green region and red region are extremely insufficient. Particularly, a color component of a long wavelength side of the visible light region, that is, the component of red color is insufficient. Therefore, the emission looks slightly bluish white, although it looks white at first glance. Accordingly, when the aforementioned white LED lighting unit is used as the general lighting unit, as for a red object, it looks significantly dull red and deteriorated in color reproducibility, thereby being improper as a general lighting unit.
In order to improve the color rendering properties of the aforementioned white LED lighting unit synthesizing the blue emission and the yellow emission, the white LED lighting unit has been developed with improved color rendering properties, having a structure in which the blue LED, the phosphor emitting yellow light under the excitation of the blue light emitted by the LED, and the phosphor emitting red light under the excitation of the blue light emitted by the LED are combined, namely, the blue emission, the yellow emission, and the red emission are combined, to obtain a white emission. The white LED lighting unit thus constructed to obtain the white light by combination of the blue light emitting LED and a plurality of phosphors, is capable of obtaining an arbitrary emission color in addition to the white light, according to the combination and mixing ratio of the phosphors.
For example, patent document 1 discloses the phosphors containing nitrogen such as silicon nitride-based phosphors including (Ca, Sr, Ba)2Si5N8:Eu, as a red phosphor having an improved excitation band in the long wavelength side, and capable of obtaining an emission peak with a large half value width. In addition, the light emitting device having improved color rendering properties is proposed, by combining the blue emitting LED, the red phosphor emitting red light under the excitation of the blue light emitted by the LED, and yellow light emitting YAG:Ce and so forth. However, when increasing the emission intensity of the white LED lighting unit, a phenomenon of so-called “color shift” occurs, which is a problem of making it impossible to obtain a target emission color. Therefore, in order to solve the phenomenon of the “color shift”, patent document 2 proposes the phosphor mixture used by mixing the phosphors having approximately the same deterioration in emission characteristic at temperature rise. (Patent document 1) International Publication WO2004/039915 A1 Pamphlet (Patent document 2) Japanese Patent Laid-Open No.2004-235598
However, according to the study by the inventors of the present invention, it is found that even when green to orange phosphors and the silicon nitride-based red phosphor containing nitrogen shown by the patent document 2 are mixed, and the phosphor mixture thus obtained and the light emission element (LED) are combined, to thereby manufacture the light-emitting device, and by feeding an electric current to the light emitting device, the white light having a low correlative color temperature is obtained, the degree of the color shift is yet large, and the light with high color rendering properties is not obtained.
In view of the above-described circumstances, the present invention is provided, and an object of the present invention is to provide the phosphor mixture used in the light emitting device such as the white LED lighting unit, which is the phosphor mixture that hardly generates the color shift even when strong emission is obtained from the LED used for the white LED lighting unit or the like, and the light emitting device having the phosphor mixture and the light emitting part that hardly generates the “color shift”.